The invention relates to a method and apparatus for spectroscopic quantitative analysis of gases in gas mixtures.
The objects of the present invention are attained by a method of and apparatus for spectroscopic quantitative analysis of gases in gas mixtures with which monochromatic light of at least two wavelengths is used in periodically alternating fashion, the light of at least one wavelength being characteristic and the light of at least one other wavelength being uncharacteristic of the transmission of the gas to be determined, wherein the transmission generates electrical signals which comprise harmonic components of at least one uneven Fourier frequency of an emission period the amplitude of which is proportional to the concentration of the gas to be determined in the gas mixture and which vanishes together with the concentration.
The quantitative analysis of gases in gas mixtures is made on the basis of the absorption of radiation having a wavelength which is characteristic of the gas in question. Molecular gases dispose in particular of a pronounced characteristic for that purpose in the far infrared region.
Spectral distinction of the characteristic absorption by the gas to be analyzed from background radiation or from the uncharacteristic radiation damping along the transmission path is obtained by periodically sequencing the emission from a suitable source of radiation according to conventional analytical procedures at a wavelength which is characteristic of the particular gas and at a respective adjacent unspecific wavelength, with respective interruptions.
Lasers offer themselves as radiation sources; a dye laser is suitable for use in the visible region and a suitable molecular gas laser in the infrared region. The latter utilize the vibrational-rotational transitions of the respective gases. The molecular gas laser is set to the wavelengths of the respective desired emission lines by means of a resonator mirror or optical grating supported by a micropositioning means. The microposition can be controlled by electrical signals for generating the periodic emission sequence at different wavelengths.
In this context the quantitative analysis of ammonia in flue gases is of industrial-scale importance. Flue gases are produced with every atmospheric combustion and together with them necessarily also nitrogen oxides NO.sub.x. The latter, on the one hand, can be reduced to products which are free of noxious components by feed ammonia. On the other hand, the so-called "slip", in other words a noticeable excess of ammonia is undesired.
By law of nature one line each of the vibration-rotation bands of ammonia and .sup.13 C.sup.16 O.sub.2 coincide at wavelength 9.89 .mu.m within the line widths at normal or operating conditions of a furnace. Therefore, the absorption of the radiation of a tuned .sup.13 C.sup.16 O.sub.2 laser in a gas mixture informs about the content of ammonia.
If the gas to be analyzed--in the instant case ammonia in flue gas--is present in traces, the specific and unspecific absorptions in the laser emission sequence differ only little so that the transmission signals of the sequence are only little different from one another and, in addition, contain incoherent signal noise contributions from the laser emission, the noise in the measuring section and in the detector means. These circumstances make it necessary to apply long signal integration times in order to obtain the signal reliability as required, for instance, in industrial applications. That, on the other hand, means that the arrangement must meet high operational stability requirements.